Method for downhole sulfur removal and recovery

ABSTRACT

Sulfur sought to be removed from deposits formed at a subterranean gas producing well are dissolved in a high boiling point relatively pure and low cost non-aqueous solvent. The dissolved sulfur is then removed from the solvent by lowering the temperature of the solution to precipitate the sulfur which is then separated and recovered as a relatively pure product. After additional processing the regenerated solvent is injected back down the well hole to dissolve additional sulfur and reutilized in successive cycles of the process.

RELATED APPLICATION

This application claims priority from U.S. Provisional patentapplication Ser. No. 60/604,510 filed Aug. 26, 2004.

FIELD OF INVENTION

This invention relates generally to production of natural gas fromsubterranean wells, and more specifically relates to the removal andrecovery of sulfur which may be present when natural gas is producedfrom sour and other gas wells.

BACKGROUND OF INVENTION

When natural gas is produced from gas wells and particularly from sourgas wells, solid sulfur or sulfur vapor is often generated if the gas isquite hot and accumulates or condenses on the well tubing itself andover time builds up thus reducing the gas flow or even plugging thewell. This accumulated sulfur must be removed. One solution used in thepast is application at the well of a sulfur scavenger composition whichdissolves the sulfur, with the scavenger solution containing the sulfurthen being discarded. This solution while it can be perfectly effective,is also both economically wasteful and environmentally detrimental.

In U.S. Pat. No. 4,322,307 to Roland Kettner a process is disclosed fortreating a sour gas well in which an alkyl naphthalene base sulfursolvent is mixed with an oil carrier and circulated into the well andthen recovered from the well mixed with the produced gas. After furtherprocessing the sulfur is crystallized from the solvent under suitablecooling conditions and the solvent recovered for re-use. The sulfur fromthe separation is described in the patent as being delivered to asuitable sulfur disposal facility. The aforementioned carrier oil whichis mixed with the solvent is needed so that a sufficient densitydifferential can be achieved in order to readily accommodate separationof the solvent from entrained water contained in the production fluid.The cited patent also contains a discussion of additional prior artprocesses and methodology that have been used in the past for removal ofthe sulfur by means of various solvents and the like.

The process of the U.S. Pat. No. 4,322,307 patent while potentiallyuseful is based on a solvent with a vapor pressure that is high enoughto cause losses to the gas phase, when compared to the solvents used inthe present invention. The U.S. Pat. No. 4,322,307 solvent used is lesseconomical and less environmentally friendly than the solvents of thepresent invention due to the higher vapor losses of the solvents used inthat patent.

SUMMARY OF INVENTION

Now in accordance with the present invention the sulfur sought to beremoved from the gas producing well is dissolved in a high boilingpoint, relatively pure and low cost non-aqueous solvent, the solventfurther being more environmentally friendly than those used in the priorart. Neither does the solvent used in the invention require in its usemixing with a carrier oil. The sulfur is removed from the solvent bylowering the temperature of the solution to precipitate the solvent,with the sulfur then being separated and used as a relatively pureproduct. The regenerated solution, then lower in sulfur concentration,is injected back down the well hole to dissolve additional sulfur andmay then be reutilized in subsequent cycles of the process.

BRIEF DESCRIPTION OF DRAWINGS

The invention is diagrammatically illustrated, by way of example, in thedrawing appended hereto, in which:

The FIGURE is a schematic block diagram of a system for well head sulfurremoval operating in accordance with the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

In accordance with the present invention a sulfur solvent is utilizedwhich has a high solubility for elemental sulfur and a low vaporpressure, thereby reducing the chemical consumption costs. Thenon-aqueous solvent is selected from the group consisting of diphenylethers, dibenzyl ethers, terphenyls and alkylated terphenyls,diphenylethanes and alkylated diphenylethanes, and mixtures thereof.Preferably the selected member or members should have a boiling pointabove 290° C. (at one atmosphere). Preferably further, the solventconsists of one or more diphenyl ethers. The solvent is injecteddownhole at a typical pressure in the range of atmospheric to 4,000 psi.No mixing of the solvent with a carrier oil is needed. Preferably,relatively high pressures in the range of 2000 to 4000 psi are used. Thesolvent is heat stable at well over 260° F. and will readily dissolveany elemental sulfur deposits. The sulfur laden solvent is transportedwith the gas to the wellhead where it is separated from the gas in ahigh pressure gas liquid separator. The solvent and any associated wateris flashed and separated prior to regeneration and crystallization. Thesulfur is crystallized and separated from the solvent and theregenerated solvent is reinjected into the well. Any vaporization lossesare made up by using a solvent make up system.

Referring to the appended schematic FIGURE, a system 10 in accordancewith the present invention is shown. Wellhead gas 12 is taken from awell at an underground subterranean formation and includes the gas whichis usually (but not necessarily) sour along with the solvent which hasbeen previously injected and may contain residual quantities ofdissolved sulfur. The extracted gas and solvent are provided to a highpressure gas-liquid separator 14. The product gas is taken at 16 for itsintended ultimate use. The solvent and dissolved sulfur proceeds fromseparator 14 via line 18 to a flash tank separator 20. The gas componentis flashed there and exits at 22 where it can be recompressed or flared.Sour water is taken as one separated product at line 24. The solventcontaining the dissolved sulfur proceeds via line 26 to a loop 28 whichincludes a recirculation pump 30 and a heat exchanger 32. The cooledsolution enters a crystalizer tank 34 where the sulfur is crystallizedand the solvent then returned via line 36 to a surge tank 38 heated witha steam line 40. The solvent proceeds from surge tank 38 via line 41 tothe main injection pump 42 and then downhole at 44. A chemical make upstorage tank is provided at 46 which via a metering pump 48 providesmake up solvent to tank 38 by line 50.

The sulfur (slurry) taken from crystalizer tank 34 proceeds via line 52and a filter feedpump 54 which pumps the sulfur slurry to a pressurefilter 56. Sulfur cake 60 is separated at that point and removed via 58as relatively high purity sulfur. A solvent recovery system with cleansolvent tank 62 provides clean wash solvent and wash water by line 64 tothe pressure filter. Dirty wash solvent and wash water proceeds from thefilter to tank 62 via line 66. Oily water blow down is taken at line 68from the solvent tank and discarded or further treated for purification.

It should be noted that the sulfur crystallization process per sedescribed herein is used with suitable solvents in the processesdisclosed in several patents to David W. DeBerry et al., such as U.S.Pat. No. 6,416,729. The formation of large sulfur crystals by means ofthe present invention provides sulfur that is up to 99 percent pure andleaves the filter with only 2 to 5 percent water. In consequence thesulfur product is relatively dry and is of sufficient purity that it canbe used in agricultural applications, blended into Claus sulfur (ifavailable) or sent to a land fill as non-hazardous waste.

While the present invention has been set forth in terms of specificembodiments thereof, the instant disclosure is such that numerousvariations upon the invention are now enabled to those skilled in theart, which variations yet reside within the scope of the presentteaching. Accordingly, the invention is to be broadly construed andlimited only by the scope and spirit of the claims now appended hereto.

1. A method for removal and recovery of sulfur which may be present whennatural gas is produced from sour and other gas wells, comprising:dissolving the sulfur in a high boiling point, relatively pure and lowcost non-aqueous solvent selected from one or more members of the groupconsisting of diphenyl ethers, dibenzyl ethers, terphenyls and alkylatedterphenyls, diphenylethanes and alkylated diphenylethanes, and mixturesthereof; removing the sulfur from the solvent by lowering thetemperature of the solvent to precipitate the sulfur; and separating theprecipitated sulfur as a relatively pure product.
 2. A method inaccordance with claim 1, wherein the said solvent has a boiling point ofabove 290° C. at one atmosphere.
 3. A method in accordance with claim 1,wherein the said solvent, following the separation of sulfur therefromis recirculated to the said wells to dissolve additional sulfur for saidremoval and recovery.
 4. A method in accordance with claim 2, whereinthe solvent consists of one or more diphenyl ethers.
 5. A method inaccordance with claim 4, wherein the said solvent for dissolving thesulfur is injected into a said well at pressures from atmospheric toabout 4,000 psi.
 6. A method in accordance with claim 5, wherein saidpressure is in the range of 2,000 to 4,000 psi.